Recently, an infrared camera that detects infrared rays radiated from an object to perform object recognition has been widely used for the purpose of security in darkness. In this connection, when using the infrared camera for the purpose of security, it is necessary to obtain a clear image, and for that purpose, it is necessary to accurately form infrared light transmitted through a lens group in a lens unit into an image on a detection surface of an infrared sensor. To achieve cost reduction, however, so-called “bolometer type” is often used. Bolometer-type infrared sensor is not easily influenced by thermal convection and the like that causes degradation of detection accuracy and is vacuum-locked in a container composed of metal or the like. Therefore, a position of the detection surface can easily deviate during the manufacturing process, and it is not possible to check an internal state of the container from the outside. Even a slight deviation of the detection surface of the infrared sensor results in error where the position of the rear-side focus of the lens unit does not match the detection surface. That makes it becomes difficult to automatically control the position of the focus adjustment lens, and it becomes impossible to obtain a clear image.
To cope with the above problem, for example, Patent Literature 1 discloses a rear-side focus adjustment system of infrared camera and the method to adjust the rear-side focus, in which focusing is possible even if there are individual differences in the position of the detection surface of the infrared sensor due to vacuum processing. More specifically, the rear-side focus adjustment system for infrared cameras disclosed in Patent Literature 1 adjusts a reference position of a lens group in an optical axis direction, with the position of the detection surface position of the infrared sensor as a rear-side focus, based on detection surface position information from the outside received by means of a lens unit (see claim 1 and the like in Patent Literature 1).